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Pornography, though prevalent in the modern world, still isn't the sort of thing one expects to see while waiting in traffic behind a cop car. That's especially true at the busiest downtown intersection of a wealthy Chicago suburb like Wheaton, Illinois, best known for being the home of an evangelical Christian college once attended by Billy Graham.

But pornography is exactly what an irate Wheaton resident named Robin said he witnessed. On the morning of September 18, 2013, while sitting in his conversion van and waiting for a stoplight to change, Robin found himself directly behind Wheaton Police squad car 359. The height of his seat gave him a perfect view through the rear windshield of the squad car, and he could see the car's mobile data computer displaying "scrolling pictures of completely naked women."

The light turned green, the cop turned onto Main Street, and Robin drove home to fume for more than a week. He refused to report the incident because he didn't believe the police would actually investigate one of their own, but on September 27, after urging from his wife, he fired off an e-mail to Wheaton Mayor Mike Gresk. (Ars Technica acquired all relevant documents about the case from the city and the police department through a public records request.)

"Dear Mayor Gresk," it began, "I was trying to forget about this, but haven't been able to forget waiting behind a Wheaton Police car at the corner of Main and Front a few days ago and watching the 'officer' watching porn on his laptop. Very revealing. It certainly inspires me to pay my taxes for servants like this..."

On September 30, a City Hall employee forwarded the message to Wheaton's police chief, Mark Field, and an internal investigation began the next morning. Lieutenant Robert Miller of the Patrol Division took an official complaint from the angry resident and then began pulling records to see if the story held up. According to the duty roster, the day watch occupant of squad car 359 at the time of the complaint was Officer Thomas Sommerfield, an $87,000-a-year patrolman with more than 20 years of police experience.

Would a veteran officer like Sommerfield really have pornography showing on his squad car's laptop as he drove around town? A simple search would show whether Robin's story had its details right. Each city-owned police car comes equipped with an automatic vehicle location (AVL) system that uses GPS to log and report the car's location at all times.

Miller pulled the AVL records for squad car 359 and scrolled to the time and date of the complaint. Bam—on September 18 at 9:37am, squad car 359 was at GPS coordinates 41.86488 -88.1068, the corner of Main and Front streets in downtown Wheaton. The squad car had stopped there for three minutes before turning onto Main Street, exactly as the complaint had alleged.

Sommerfield was at that moment on patrol in car 359, his usual vehicle. Contacted by radio, he was asked to take his car out of service when he ended his shift at 3:00pm. The vehicle was secured in a parking lot at the Public Works department overnight.

The following morning, October 2, Miller sent Sergeant William Murphy over to Public Works to pull the car's onboard computer and its Verizon wireless data card. The devices were then placed in evidence locker #41 at the police department.

Sommerfield knew that something unusual had happened when he arrived at work that morning and was assigned a different car. He waited for two days without getting more details and then, during his lunch break on October 4, Sommerfield approached IT employee Pat Sinks at the counter in front of the police department's records area. He asked Sinks what had happened to the computer from car 359. Sinks told him it had been pulled for a forensic exam.

Sommerfield walked away, but five minutes later he tracked Sinks down again, this time in the IT office. Sommerfield knocked on the door, went in, and sat down.

Sinks told him that an "investigation" was underway. Sommerfield, who according to Sinks looked exceptionally nervous throughout the conversation, asked what the investigation was about.

"Porn," Sinks replied.

Sommerfield left, finished his shift, and returned home. At 5:32pm, he picked up the phone and called Sergeant Murphy. Murphy confirmed that he had pulled car 359's computer at the request of Lieutenant Miller but said he didn't know why it had been seized.

Sommerfield said that he was sick to his stomach over the whole situation. He asked Murphy if the two could speak "as friends."

Enlarge/ The GPS log from Sommerfield's car, showing how often the vehicle is tracked. The highlighted lines are the time at which Robin noticed the porn.

Porn, in spreadsheet form

Who watches the watchers? Technology does—to a point.

The computer system standard in Wheaton patrol vehicles includes constant GPS tracking along with a connection to the local DuPage county computer-assisted dispatch (CAD) system, known as DuComm. DuComm tracks an officer's activity throughout the day, letting dispatchers see which officers are available to respond to 911 calls, for instance, and which are occupied with other incidents. Many of these assignments are generated centrally, but officers can also generate their own field-initiated incidents through the system. The records are stored, making it trivial for investigators to find out what Sommerfield said he was doing when he was actually watching porn.

Enlarge/ When you watch enough porn that investigators have to compile spreadsheets, you know you're in trouble.

On October 10, the computer from squad car 359 was pulled from evidence and handed over to the nearby Burr Ridge police department for examination. A Burr Ridge analyst pulled the computer's hard drive and attached it to a write-blocker, which he then connected to an examination computer running the forensic tools Internet Evidence Finder and EnCase.

The tools quickly showed "25 pictures depicting nudity and/or graphic sexual activity" in unallocated hard drive space—suggesting that they had been deleted but not actually purged from the disk. Four "Web video fragments depicting sexual activity" also turned up in unallocated space. Website cookies on the hard drive showed that Sommerfield had visited numerous pornographic websites, including amateurs-gone-wild.com "hundreds of times." Internet Explorer cache files still showed "thousands" of bits of pornographic content.

Combining all of this data with the GPS log and the dispatch records suggested that Sommerfield had repeatedly watched porn during "field-initiated incidents" when his car was parked near local elementary schools.

For instance, on June 6, 2013—the last day of school for local students—Wheaton police were under instructions to conduct "high-visibility patrol in the area surrounding schools" in order to keep kids safe. Sommerfield had been assigned Wheaton's "beat five" that day, which included Lowell Elementary School. At 8:34am, he logged the field-initiated incident "9033-EXTRA PATROL" at the school's address. Car GPS records showed that the vehicle was stationary outside the school from 8:36am to 9:13am.

But the recovered computer records show that Sommerfield wasn't just keeping an eye out for student safety. Instead, he used his in-car computer's Web browser to visit pornography and escort websites from 8:39am to 8:49am. During this period, the analyst report notes, Sommerfield "did not initiate any other police duties (e.g. traffic stop, suspicious person, traffic control, parking enforcement)."

At 9:14am, he moved his squad car several minutes' drive away to—seriously—Fapp Circle, where he accessed another pornographic website. He moved his car again and at 9:41am looked at porn from a third location.

This kind of intermittent porn-watching was apparently typical. When school started up in August after summer break, Sommerfield was again tasked with "high-visibility patrol," he again created a field-initiated incident, and he again parked his car outside an elementary school. He watched porn intermittently from 8:24am to 8:47am, when he was sent elsewhere to help another officer with a suspicious vehicle.

At 11:43am, just in time for kindergarten mid-day dismissal at 11:45am, Sommerfield was at another elementary school for another "extra patrol." At 11:51am, as his car sat idle outside the school, he was visiting porn sites.

This pattern of activity was so extensive that the Burr Ridge forensic analyst wrote, "The sheer volume of information discovered was too voluminous to include in this report. This report is a mere sample of the conduct and violations of Officer T. Sommerfield."